Display screen or portion thereof with a transitional graphical user interface



FIG. 1 is a front view of the display screen or portion thereof with a transitional graphical user interface showing a first image in a sequence according to the claimed design;

FIG. 2 is a front view of a second image thereof; and,

FIG. 3 is front view of a third image thereof.

The appearance of the transition is sequential from FIG. 1 to FIG. 3. The process or period in which an image transitions to another image forms no part of the claimed design.

The shading in the figures indicate a contrast in appearance.

The dash-dot line shows the display screen or portion thereof and forms no part of the claimed design. The broken-line showing of numbers and the colon forms no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface, as shown and described. 